In one air-operated lubricant pump known in the prior art, disclosed in European patent No. EP-OS 0 039 418, a rotary slide valve that alternates the side of the piston on which compressed air is introduced is connected to the reciprocating piston rod of the pump by means of a slideway, and every time the piston rod moves to one of its end positions the rotary slide valve changes its angular position through the pressure of a hairpin spring. Between the end positions of the piston rod, the rotary slide valve is kept in its desired angular position by means of a control bar. Compressed air is introduced into a tubular chamber in which the slide valve moves, and is routed through ports in a front wall of the tubular chamber.
One of the shortcomings of this prior art device is that a hairpin spring, which relies on a single bend, is more susceptible to failure than a spring of the type employed in the present invention, i.e., a helical compression spring. A second shortcoming is that in this prior art device compressed air flows constantly around the hairpin spring, and this continued exposure to compressed air decreases the useful life of the spring. Another shortcoming is that the installation of this prior art control device is quite difficult.
Another type of air-operated lubricant pump, disclosed in German patent No. 35 27 925, is one in which an axially movable slide valve is utilized to change the side of the piston on which compressed air is introduced, as the piston moves forward and backward to produce a pumping action. This prior art device is not automatically operated, but relies on manual operation of a two-position valve for introduction of compressed air into the device. The cylindrical slide valve has a complicated arrangement of air passages extending through its walls. Because of the construction of this prior art device, the reversal of direction of the reciprocating piston rod is effected by pressure differences within the slide valve, which makes a waiting period necessary for the individual air passages and chambers to be aerated and de-aerated before the piston rod reverses its direction.
The air-operated lubricant pump of the present invention avoids the indicated shortcomings of the prior art, and provides reliable, economical and automatic reciprocating movement of the piston rod with a minimum of maintenance for long periods of time. The manufacture of the device is much simpler than the manufacture of the second prior art device just discussed, because there is no necessity to cast or bore any maze-like passages through the walls of the cylindrical slide valve, as is the case with the prior art device in question.